1. Field of the Invention
This invention relates to a method and apparatus for determining the phase behavior of oil/gas mixtures at controlled high pressures and temperatures.
2. Description of the Prior Art
Many procedures associated with the production of oil and gas require detailed knowledge of the phase behavior of gas/oil mixtures at high pressures and temperatures. Contact between an oil and a gas at elevated pressures and temperatures often produces an oil phase cut with gas, and a gas phase containing often high concentrations of oil. The purpose of phase-behavior studies is to determine quantitatively the composition and volume of each phase at various pressures and temperatures. Such data is fundamental to the continuous circulation of corrosion inhibitors in deep high-pressure sour-gas wells, or to enhance oil recovery by CO.sub.2 -flooding. In the case of continuous circulation of corrosion inhibitors, this may involve measurements performed at pressures to 20,000 psig. and temperatures to 500.degree. F.
The present accepted technique for performing such studies consists of charging a small quantity of oil (say, 10-15 gm) into a high-pressure cell, and following this with a charge of gas (which may equal 150 liters of methane, carbon dioxide, hydrogen sulfide, etc., or mixtures thereof). The whole cell is then brought to the desired pressure and temperature. Data is taken by venting small quantities of the gas phase through a trap, and subsequently measuring the volume of gas vented and weighing the liquid (oil) collected. This process is conducted while the cell and its contents are nearly maintained at constant pressure and temperature.
The previously discussed techniques suffer several serious drawbacks:
(1) Since the cell that contains the oil/gas mixture is also brought to high pressure and temperature, strength of materials becomes an overriding safety consideration. Also, the presence of corrosive H.sub.2 S and CO.sub.2 gases often dictates the use of expensive alloys to prevent H.sub.2 /H.sub.2 S embrittlement. Massive, thermally controlled cells fabricated from $30-$50/lb. alloys (Hastaloy C-276, MP35N, etc.) are expensive, require extremely long delivery times, and are not generally available on a contract basis. PA1 (2) In conducting studies at high pressures and temperatures, the volume of the liquid phase is often reduced to less than 10% of the original charge. The liquid phase composition can only be estimated from the gas phase composition, and the volume of the liquid phase cannot be measured directly because of its small volume.